The present invention relates to an apparatus for measuring thickness differences in record carriers, such as bank notes and the like.
German Published Application (DT-OS) 1,474,903 discloses a bank note checking device wherein, while the bank note is moving through the checking device, the surface of the bank note is sensed by means of a sensor in order to obtain a test signal dependent on the surface contour. Via a lever transmission, the deflections of the sensor are passed on to a piezoelectric pressure transmitter, operating, in principle, like a phonograph pickup, or to a resistance strain gauge arrangement, with the deflections of the sensor generating the test signal.
Since, with this device, the possibility of damaged bank notes being caught by the sensor and, consequently, of the checking device or of the bank notes being destroyed cannot be excluded, the use of such a thickness gauge appears to be not entirely unproblematic. Since, in addition, the upper limit frequency of this device depends primarily on the spring/mass system of the arrangement, and since the small thickness differences to be measured require large magnitude gear ratios in order to obtain useful signals, the upper limit frequency of the device is relatively low, so the bank notes can only be checked very slowly.
German Published Application (DT-OS) 2,423,094 discloses another bank note checking device wherein, following the principle of the above-described device, the bank note is passed over a rotating drum, and its surface contour is sensed along at least one line parallel to the edge by means of a stationary leaf spring. Via a lever arrangement and a piezoeletric device, the deflections of the leaf spring generate signals which are proportional to the thickness of the paper. Through the use of a leaf spring, the risk of the bank notes and of the sensor being damaged must be negligibly small with this thickness gauge, but since the construction corresponds in principle to that of the first-mentioned publication, this device, too, has the disadvantages of the low processing speed.
Since both of the known checking devices sense only the surface contour of one side of a bank note, they suffer from the additional drawback that authentic changes in paper thickness cannot be distinguished from impressions where the surface of the paper is deformed, too, while the thickness is substantially the same. Therefore, counterfeit bank notes with impressed watermarks will, in all probability, be accepted by these devices as valid.